The water and methanol transport into a short-side-chain perfluorosulphonic acid ionomeric (PFSI) membrane suitable for application in proton exchange membrane fuel cells (PEMFC), namely Hyflono (R) Ion, was studied between 35 and 65 degrees C. In particular, the permeabilities of pure water, pure methanol and their mixtures at different temperatures were measured through pervaporation experiments, at various values of feed composition. Due to the presence of mutual interactions between permeants as well as among penetrants and polymeric matrix, the composition of the feed solution affects the membrane permeability in a way which cannot be predicted on the basis of permeability data of the pure liquid components alone. It has been found in particular that the presence of the water in the mixture enhances the methanol permeability, due to the positive effects of matrix plasticization and favourable energetic interactions. In turn, by considering water permeability data in the presence of a poorly permeating component such as glycol, it can be concluded that also water permeation is enhanced by the presence of methanol, although to a lower extent. The molar equilibrium uptake of pure water and pure methanol were also measured in separate sorption experiments and result the same for both components, while the estimated diffusion coefficient of water is 3.1-3.5 times that of methanol. The activation energies for the relevant transport properties in the range between 35 and 65 degrees C were calculated, both for pure penetrants as well as for the penetrants in mixture, thus determining the deviations from the pure component behaviors. (C) 2008 Elsevier B.V. All rights reserved.